The role of climatic variables on nest evolution in tanagers

Abstract Avian nests are fundamental structures in avian reproduction and face strong selective forces. Climatic conditions are likely to have shaped the evolution of specific nest traits, but evidence is scarce at a macroevolutionary level. The Thraupidae family (commonly known as tanagers) is an ideal clade to understand the link between nest architecture and climate because it presents wide variation in nest traits. To understand whether climatic variables have played a role in the diversification of nest traits among species in this family, we measured nests from 49 species using museum collections. We observed that dome‐nesting species are present in dryer and hotter environments, in line with previous findings suggesting that domed nests are a specialisation for arid conditions. We also found evidence that nests with thicker walls are present in locations with lower precipitation and that solar radiation can influence the shape of domed nests; birds tend to build shorter and narrower domes in areas with high levels of solar radiation. Open nest architecture is also potentially influenced by wind speed, with longer and deeper nests in areas characterised by strong winds. Our results support the hypothesis that different climatic variables can drive the evolution of specific aspects of nest architecture and contribute to the diversity of nest shapes we currently observe. However, climatic variables account only for a small fraction of the observed structural variation, leaving a significant portion still unexplained.


Nest measurements
Nest height: In open nests, the measurement of length extended from the entrance (the rim of the cup) to the bottom of the nest, while in domed nests, height was determined from the top of the nest to the bottom.
Cup depth: For both nest types, the depth of the cup was measured from the entrance to the bottom, employing a slender ruler.
Total nest diameter: In open nests, width was defined as the broadest part of the nest, while in domed nests, width was gauged along the base of the entrance hole.
Internal (entrance for domed nests) diameter: In open nests, internal diameter was ascertained by measuring the internal diameter both vertically and horizontally, followed by an averaging of these two measurements.In domed nests, the entrance diameter was ascertained by measuring the entrance diameter both vertically and horizontally, followed by an averaging of these two measurements.
Wall thickness: Determined by measuring the nest's thickness at four distinct points (evenly spaced at 90 degrees from each other around the entrance hole) and then averaging these four measurements.
All measurements besides cup depth were taken using digital calipers and all nests were photographed in a standardized manner.

Collinearity
A subset of the nest measurements was selected aLer checking for correlaMon between variables (Figure S1).A threshold of 0.7 was chosen to discard one of the highly correlated variables, resulMng in the nest width variable in the open nest subset, being discarded.We used the R package "Performance" and the command check_model to check for collinearity between predictor variables.The degree of collinearity is determined using the variance inflaMon factor (VIF), where a VIF value of less than 5 is deemed acceptable for individual predictors (Lüdecke et al., 2019).The number of response variables was chosen on the basis of VIF value, when a high correlaMon between two variables was highlighted one of the variables was discarded.In both open and domed nest subsets, enough variaMon in the predictors in all models was detected to avoid mulMcollinearity issues.
One nest in the domed nest category and one in the open nest category were discarded as they were considered outliers.
Table S1.Climatic variables included in this study (Muñoz Sabater, 2021).(C -D) using climaMc data using an average of the months of the breeding period for those nests whose month of collecMon was not provided (in purple) and using climaMc data for the month during which most nests of that species were likely to be built (in green).Predictors are Temperature (PC), PrecipitaMon (PC), RadiaMon (PC), mean wind speed, and the log-

Climate variable
Eastward component of the wind at a height of ten meters above the ground.Given the u and v components, the magnitude of the wind vector can be determined using the Pythagorean Theorem: wind speed= sqrt (10u 2 + 10v 2 ).

Figure S6 .
Figure S6.Posterior samples from the brms models for domed nests (A -B) and open nests log(mass)) of the species.Posterior samples for domed nests (A -B) exclude the nest belonging to the species Cer4dea olivacea.

Table S2 .
Loadings and proportion of variance obtained from the Principal ComponentAnalysis on temperature, precipitation, and solar radiation variables.

Table S4 .
BRMS results for open nests excluding the nest belonging to the species Sicalis flaveola.Predictors are Temperature (PC), PrecipitaMon (PC), RadiaMon (PC), mean wind speed (mean wind), and the log-transformed mass (log(mass)) of the species.

Table S6 .
BRMS results for domed nests excluding the nest belonging to the species Cer4dea olivacea.Predictors are Temperature (PC), PrecipitaMon (PC), RadiaMon (PC), mean wind speed (mean wind), and the log-transformed mass (log(mass)) of the species.

Table S8 .
BRMS results for open and domed nests combined excluding the nest belonging to the species Cer4dea olivacea and the nest belonging to the species Sicalis flaveola.Predictors are Temperature (PC), PrecipitaMon (PC), RadiaMon (PC), mean wind speed (mean wind), and the log-transformed mass (log(mass)) of the species.